A-level Chemistry Essay



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A-level Chemistry - Essay

by T. Fung







B-2 Carbonyl compounds contain C=O and alkenes contain C=C. Although both compounds are unsaturated and undergo addition reaction, they react towards different reagents, nucleophiles or electrophiles. Account for these differences using electronic theory.
Your essay should emphasize the mechanism of the typical reactions of carbonyl compounds and alkenes.

(20 marks)




Keys in Subheadings
I Electronic structures of C=O & C=C (4 marks)

- s and p bonds; electron clouds; bond polarity; difference in EN; I-effect; M-effect; delocalization of p electrons; electrophiles; nucleophiles;


II Typical reactions of carbonyl compounds (3 marks)

- AN reaction with general mechanism

- Relative reactivity

- Addition of HCN and NaHSO3


III Typical reactions of alkenes (3 marks)

- AE reaction with general mechanism

- Relative reactivity

- Addition of HBr and Br2


Answer to B-2.
I Electronic structures of C=O & C=C (4 marks)

Characteristics of >C=O


1. The carbonyl carbon atom undergoes sp2 hybridization with trigonal planar structure.

2. The unhydridized p-orbitals of C-atom overlaps with a p-orbital of O-atom to form a p-bond.



3. Due to inductive effect & mesomeric effect, the C=O bond is polarized resulting in a permanent dipole (with C is d+ and O is d-)

Structure of carbon-carbon double bond >C=C<

1. The C-atoms in double bond are trigonal planar.

2. Two negatively charged p-electron clouds lie above and below the plane of atoms.



3. The p-electrons are much exposed than those in the s bond and are therefore more vulnerable to any attacking species, particularly electrophiles.
Comparison of C=O & C=C





Carbonyl group (C=O )

C=C double bond (C=C)

Structure / Shape

C-atom undergoes sp2 hybridization and it is trigonal planar.



C-atom undergoes sp2 hybridization and it is trigonal planar.



Bonding

s and p bond

Because of the presence of unsaturated bond, it undergoes addition.



s and p bond

Because of the presence of unsaturated bond, it undergoes addition.



Polarity

C=O bond is polar.

Oxygen is more electronegative and withdraws electron density by I & M-effect



C=C bond is non-polar.

Typical reaction

Nucleophilic addition (AN)

The positive carbonyl carbon will attract nucleophiles (e.g. CN-) and it undergoes nucleophilic addition.



Electrophilic addition (AE)

The negative p electron clouds above and below the double bond will attract electrophiles (e.g. H+) and undergoes electrophilic addition.



II Typical reactions of carbonyl compounds (3 marks)


Mechanism of nucleophilic addition

Because the carbonyl group (>C=O) is planar, this part of the molecule is



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